Owing to its stability and environment-friendly properties, the SrAl₂O₄:Eu²⁺, Dy³⁺ (SAOED) phosphor has attracted major scientific interest.

This article proposes a novel folded stepped-impedance hairpin-ring resonator and designs a compact sixth-order dual-passband high-temperature superconducting (HTS) filter with a comprehensive wide-stopband response. The design employs a stepped-impedance resonator and a hairpin-ring resonator. The resonator uses a dual-feed structure to achieve external coupling for both passbands. It introduces a U-shaped secondary coupling line between the resonators to flexibly control the internal coupling, enabling independent control of the frequency and bandwidth of the two passbands. Using this method, we designed a sixth-order dual-passband HTS filter. The first passband center frequency is 1.9 GHz with a bandwidth of 40 MHz (2.1% ), and the second passband center frequency is 2.8 GHz with a bandwidth of 70 MHz (2.5% ). The filter was fabricated on a YBa₂Cu₃O₇-δ, (YBCO) HTS thin film on a MgO substrate sized 35 mm × 14.5 mm (0.57λ_g × 0.23λ_g). Measurement results showed good agreement with simulation results, showing excellent passband performance. The return loss is better than 20 dB, and the insertion loss is less than 0.04 dB. The out-of-band rejection is better than 50 dB and can reach as high as 9 GHz.

By employing the synthesis method of the generalized Chebyshev function filter and the frequency-domain symmetric transformation of the transmission and reflection function response, the transformation from a single-passband filter to a symmetric dual-passband filter in the frequency domain is successfully achieved. The cascaded triplet (CT) unit features flexible control over transmission zeros (TZs), and the cascaded quadruplet (CQ) unit can introduce TZs in pairs in a single step. Based on the topology of CT and CQ units, a typical topological structure model and the corresponding coupling matrix of the dual-passband filter with specified TZ positions are extracted by the coupling matrix analysis method. A dual-passband high-temperature superconducting bandpass filter is designed with a folded compact dual-spiral microstrip resonator based on the afore mentioned design method. The filter has center frequencies of 2495.75 MHz and 2525.75 MHz, with two equal relative bandwidths of 0.86%@f ₀ and four TZs. The measured results of the filter at a temperature of 77 K show excellent sideband roll-off of ⩾12.8 dB MHz⁻¹ and minimal insertion loss of ⩽0.2 dB, which is consistent with the theoretical model and full-wave electromagnetic simulation frequency response. Thus, the effectiveness of the proposed topology and design method in achieving dual-passband characteristics and introducing in-band TZs is validated. Furthermore, this study provides an effective approach for designing high-performance ultra-narrowband dual-band filters.

High-performance filters based on high-temperature superconducting (HTS) thin film technology have been developed for 5G mobile communications applications. However, a design that enables the resonant frequencies and passband bandwidths of the dual-band bandpass filter to be easily specified is lacking. This paper presents the design of a central load stub step impedance resonator based on a T-type resonator. The feasibility of achieving a wide stopband was analyzed using a structural diagram. The filter incorporates a dual-path coupling structure to adjust the internal coupling and a dual-feed external coupling structure to control the external quality factor. A fourth-order dual-passband filter was then fabricated on a double-sided YBCO HTS thin film with the size of 15.7 mm × 9.4 mm (0.47 λ _g × 0.28 λ _g ) and tested at 55 K. It achieved center frequencies of 3550 MHz (2.8% bandwidth) and 4850 MHz (2.0% bandwidth), meeting the N78 and N79 bands of 5G technology. The filter also generated four transmission zeros on both sides of the two passbands and achieved an out-of-band suppression greater than 80 dB, demonstrating excellent performance. The stopband rejection was greater than 50 dB for frequencies of up to 11 GHz. Because of its ease of use, design will facilitate the adoption of HTS filters in communications systems.

This paper presents a miniaturized quadband high-temperature superconducting (HTS) bandpass filter based on a multistub-loaded resonator. Owing to the symmetrical structure of the proposed resonator, the odd- and even-mode analysis method can be used to derive its resonance frequencies. First, the frequency ratio between passbands may be significantly changed by adjusting the impedance ratio and fold spacing of the resonator. Moreover, the proposed filter exhibits well-controlled center frequencies and bandwidths for the four passbands, resulting in a considerably enhanced design possibility of quadband bandpass filters. A low insertion loss within each band is obtained using HTS technology. Additionally, the proposed filter was etched on an MgO substrate with a thickness of 0.5 mm and combined with YBCO thin films for demonstration. The results obtained were highly consistent with those of the simulation, validating the effectiveness and viability of the proposed method.